// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_SEQUENCE_TOKEN_H_
#define BASE_SEQUENCE_TOKEN_H_

#include "base/base_export.h"
#include "base/macros.h"

namespace base {

// A token that identifies a series of sequenced tasks (i.e. tasks that run one
// at a time in posting order).
class BASE_EXPORT SequenceToken {
public:
    // Instantiates an invalid SequenceToken.
    SequenceToken() = default;

    // Explicitly allow copy.
    SequenceToken(const SequenceToken& other) = default;
    SequenceToken& operator=(const SequenceToken& other) = default;

    // An invalid SequenceToken is not equal to any other SequenceToken, including
    // other invalid SequenceTokens.
    bool operator==(const SequenceToken& other) const;
    bool operator!=(const SequenceToken& other) const;

    // Returns true if this is a valid SequenceToken.
    bool IsValid() const;

    // Returns the integer uniquely representing this SequenceToken. This method
    // should only be used for tracing and debugging.
    int ToInternalValue() const;

    // Returns a valid SequenceToken which isn't equal to any previously returned
    // SequenceToken.
    static SequenceToken Create();

    // Returns the SequenceToken associated with the task running on the current
    // thread, as determined by the active ScopedSetSequenceTokenForCurrentThread
    // if any.
    static SequenceToken GetForCurrentThread();

private:
    explicit SequenceToken(int token)
        : token_(token)
    {
    }

    static const int kInvalidSequenceToken = -1;
    int token_ = kInvalidSequenceToken;
};

// A token that identifies a task.
//
// This is used by ThreadCheckerImpl to determine whether calls to
// CalledOnValidThread() come from the same task and hence are deterministically
// single-threaded (vs. calls coming from different sequenced or parallel tasks,
// which may or may not run on the same thread).
class BASE_EXPORT TaskToken {
public:
    // Instantiates an invalid TaskToken.
    TaskToken() = default;

    // Explicitly allow copy.
    TaskToken(const TaskToken& other) = default;
    TaskToken& operator=(const TaskToken& other) = default;

    // An invalid TaskToken is not equal to any other TaskToken, including
    // other invalid TaskTokens.
    bool operator==(const TaskToken& other) const;
    bool operator!=(const TaskToken& other) const;

    // Returns true if this is a valid TaskToken.
    bool IsValid() const;

    // In the scope of a ScopedSetSequenceTokenForCurrentThread, returns a valid
    // TaskToken which isn't equal to any TaskToken returned in the scope of a
    // different ScopedSetSequenceTokenForCurrentThread. Otherwise, returns an
    // invalid TaskToken.
    static TaskToken GetForCurrentThread();

private:
    friend class ScopedSetSequenceTokenForCurrentThread;

    explicit TaskToken(int token)
        : token_(token)
    {
    }

    // Returns a valid TaskToken which isn't equal to any previously returned
    // TaskToken. This is private as it only meant to be instantiated by
    // ScopedSetSequenceTokenForCurrentThread.
    static TaskToken Create();

    static const int kInvalidTaskToken = -1;
    int token_ = kInvalidTaskToken;
};

// Instantiate this in the scope where a single task runs.
class BASE_EXPORT ScopedSetSequenceTokenForCurrentThread {
public:
    // Throughout the lifetime of the constructed object,
    // SequenceToken::GetForCurrentThread() will return |sequence_token| and
    // TaskToken::GetForCurrentThread() will return a TaskToken which is not equal
    // to any TaskToken returned in the scope of another
    // ScopedSetSequenceTokenForCurrentThread.
    ScopedSetSequenceTokenForCurrentThread(const SequenceToken& sequence_token);
    ~ScopedSetSequenceTokenForCurrentThread();

private:
    const SequenceToken sequence_token_;
    const TaskToken task_token_;

    DISALLOW_COPY_AND_ASSIGN(ScopedSetSequenceTokenForCurrentThread);
};

} // namespace base

#endif // BASE_SEQUENCE_TOKEN_H_
